LCP, FCP, FP timing is not accurate

[Dante-dan]

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Self-hosted/on-premise

Which package are you using?

@sentry/vue

SDK Version

7.18.0

Framework Version

vue 3.2.45

Link to Sentry event

No response

Steps to Reproduce

1. reload page
2. find page load event
3. the lcp fp fcp measurements are smaller than performance API

It looks like this is deliberately designed this way.. but in my opinionin, performance API provide startTimestamp meaning that from navigation Start to First Content Paint.

But normalizedValue ≈ FCP - new Date() is smaller than FCP, and it will be affected by javascript load time, specially in weak network.

This is fatal for performance optimization scenarios.

Expected Result

we should report startTimestamp to measurement's fcp/lcp/fp value, because it actual value.

Actual Result

normalizedValue ≈ FCP - new Date()

[AbhiPrasad]

Hey @Dante-dan, thanks for writing in. Please see #3019 for some more context on why we normalized the timings here.

cc @dashed.